Phenylalanine is the metabolite which accumulates in phenylketonuria (PKU), a genetic disorder. Undetected, phenylketonuria in newborns can result in brain damage and behavioral problems. Current tests to detect this severe disease are generally done in a few laboratories to which the blood samples are sent with unavoidable delay of results. Therefore, a strip test for quick on-site determination of phenylalanine levels in blood will be a valuable tool for both the detection as well as monitoring of phenylketonuria. GDA Technology has recently developed a specific thermostable Phenylalanine dehydrogenase which forms the basis for the proposed strip test (similar to Miles' of Boehringer's glucose test). This proposal (Phase I) is a feasibility research which will focus on the development of a visual strip test by incorporating GDS' Phenylalanine dehydrogenase enzyme and indicator system which will produce color proportionate to the serum concentration of phenylalanine. The goal of Phase II is to complete the development of a whole blood strip test and perform clinical trials. The program is attractive as it offers the opportunity to enhance the standard of preventive medical care by providing accurate on-site answers. Because of its simplicity the test could be performed even in underdeveloped countries.